This invention relates generally to control circuitry, and more particularly to a method and circuit for hiding a control function in a control register of a microprocessor or the like.
Often times, a memory mapped register is used to perform a control function. For example, an address register may be used to select one of several LCD display registers. It may also be true that only the least significant bits (e.g. the four LSBs of eight bits) are required to perform the desired function. If an on/off control for the entire LCD display function is required and there is no control register available, it would be awkward and inefficient to create an additional register to perform the simple on/off function.
One approach is to use one unused bit of the address register to perform the control function. For example, a "1" in the selected bit would enable the LCD function while a "0" in the selected bit would disable it. There are conditions, however, wherein it is desirable or necessary that the address register contain either all 1's or all 0's. This could, perhaps, cause the on/off bit to be forced into the wrong state resulting in the unwanted enabling or disabling of the LCD function.